The Anti-Access/Area Denial (A2AD) strategy is an effort on the part of US adversaries to deny freedom of movement to US forces. Methods include offensive cyber operations, attacks against space navigation and communications platforms, maritime missile and mine deployments, and electronic warfare. The strategic goal is to keep US forces out of areas where they would otherwise be politically and militarily relevant, and a primary facilitator of that goal is to degrade or deny the wireless communications on which command and control depend.
Wireless communications in A2AD scenarios represent a difficult challenge. Data must be communicated over distances on the order of 800 nm to keep large, high value assets out of enemy missile range. Mobile platforms with lower radar cross-sections (especially unmanned platforms) that can move into the denied perimeter must have uninterrupted communications with assets outside. As satellite waveforms are susceptible to jamming and the satellites themselves are vulnerable to space-based attacks. It is assumed that satellite communications will be denied. Over such long distances, even reliable point-to-point links using HF and VHF are difficult. These problems are partially answered by aerial layer networking, but air platforms high exposure necessitates a robust physical waveform not only for data links but for platform C2. This waveform must be particularly resilient against swept, partial-band, and narrow-band interference. The low density of assets and the desire for their undetectability implies directional waveforms will be most useful. There appears a need for a wideband, low instantaneous bandwidth, directional waveform.
Anti-jam communications use hybrids of frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS). In both cases, the idea is to widen the bandwidth of the communications waveform such that narrow-band jamming is only effective against a small portion of the waveform. The unaffected portions can still be used to communicate data. The hop sets are confined to a few hundred megahertz because changing the carrier frequency by amounts on the order of GHz takes more time as one must wait for oscillator transients to suppress. This compromises data-rate. Similarly DSSS systems are confined to similar bandwidths because they cannot occupy non-contiguous bands in frequency space and because their bandwidth is limited by the speed of the analogue-to-digital converters (ADC) in their receivers. In either case, the conventional operational anti-jam waveform looks continuous in frequency space over a bandwidth of a few hundred MHz (once time-averaged).